A broad distinction is made between general and specialized dictionaries. Specialized dictionaries include words in specialist fields, rather than a complete range of words in the language. Lexical items that describe concepts in specific fields are usually called terms instead of words, although there is no consensus whether lexicology and terminology are two different fields of study. In theory, general dictionaries are supposed[citation needed] to be semasiological, mapping word to definition, while specialized dictionaries are supposed to be onomasiological, first identifying concepts and then establishing the terms used to designate them. In practice, the two approaches are used for both types.[3] There are other types of dictionaries that do not fit neatly into the above distinction, for instance bilingual (translation) dictionaries, dictionaries of synonyms (thesauri), and rhyming dictionaries. The word dictionary (unqualified) is usually understood to refer to a general purpose monolingual dictionary.[4]

There is also a contrast between prescriptive or descriptive dictionaries; the former reflect what is seen as correct use of the language while the latter reflect recorded actual use. Stylistic indications (e.g. "informal" or "vulgar") in many modern dictionaries are also considered by some to be less than objectively descriptive.[5]

Although the first recorded dictionaries date back to Sumerian times (these were bilingual dictionaries), the systematic study of dictionaries as objects of scientific interest themselves is a 20th-century enterprise, called lexicography, and largely initiated by Ladislav Zgusta.[4] The birth of the new discipline was not without controversy, the practical dictionary-makers being sometimes accused by others of "astonishing" lack of method and critical-self reflection.[6]

The French-language Petit Larousse is an example of an illustrated dictionary.

Arabic dictionaries were compiled between the 8th and 14th centuries CE, organizing words in rhyme order (by the last syllable), by alphabetical order of the radicals, or according to the alphabetical order of the first letter (the system used in modern European language dictionaries). The modern system was mainly used in specialist dictionaries, such as those of terms from the Qur'an and hadith, while most general use dictionaries, such as the Lisan al-`Arab (13th century, still the best-known large-scale dictionary of Arabic) and al-Qamus al-Muhit (14th century) listed words in the alphabetical order of the radicals. The Qamus al-Muhit is the first handy dictionary in Arabic, which includes only words and their definitions, eliminating the supporting examples used in such dictionaries as the Lisan and the Oxford English Dictionary.[10]

The earliest dictionaries in the English language were glossaries of French, Spanish or Latin words along with their definitions in English. The word "dictionary" was invented by an Englishman called John of Garland in 1220 — he had written a book Dictionarius to help with Latin "diction".[13] An early non-alphabetical list of 8000 English words was the Elementarie, created by Richard Mulcaster in 1582.[14][15]

The first purely English alphabetical dictionary was A Table Alphabeticall, written by English schoolteacher Robert Cawdrey in 1604. The only surviving copy is found at the Bodleian Library in Oxford. This dictionary, and the many imitators which followed it, was seen as unreliable and nowhere near definitive. Philip Stanhope, 4th Earl of Chesterfield was still lamenting in 1754, 150 years after Cawdrey's publication, that it is "a sort of disgrace to our nation, that hitherto we have had no… standard of our language; our dictionaries at present being more properly what our neighbors the Dutch and the Germans call theirs, word-books, than dictionaries in the superior sense of that title." [16]

It was not until Samuel Johnson's A Dictionary of the English Language (1755) that a more reliable English dictionary was produced. Many people today mistakenly believe that Johnson wrote the first English dictionary: a testimony to this legacy.[18] By this stage, dictionaries had evolved to contain textual references for most words, and were arranged alphabetically, rather than by topic (a previously popular form of arrangement, which meant all animals would be grouped together, etc.). Johnson's masterwork could be judged as the first to bring all these elements together, creating the first "modern" dictionary.[18]

Johnson's dictionary remained the English-language standard for over 150 years, until the Oxford University Press began writing and releasing the Oxford English Dictionary in short fascicles from 1884 onwards. It took nearly 50 years to complete this huge work, and they finally released the complete OED in twelve volumes in 1928. It remains the most comprehensive and trusted English language dictionary to this day, with revisions and updates added by a dedicated team every three months. One of the main contributors to this modern dictionary was an ex-army surgeon, William Chester Minor, a convicted murderer who was confined to an asylum for the criminally insane.[19]

In 1806, American Noah Webster published his first dictionary, A Compendious Dictionary of the English Language. In 1807 Webster began compiling an expanded and fully comprehensive dictionary, An American Dictionary of the English Language; it took twenty-seven years to complete. To evaluate the etymology of words, Webster learned twenty-six languages, including Old English (Anglo-Saxon), German, Greek, Latin, Italian, Spanish, French, Hebrew, Arabic, and Sanskrit.

Webster completed his dictionary during his year abroad in 1825 in Paris, France, and at the University of Cambridge. His book contained seventy thousand words, of which twelve thousand had never appeared in a published dictionary before. As a spelling reformer, Webster believed that English spelling rules were unnecessarily complex, so his dictionary introduced American English spellings, replacing "colour" with "color", substituting "wagon" for "waggon", and printing "center" instead of "centre". He also added American words, like "skunk" and "squash", that did not appear in British dictionaries. At the age of seventy, Webster published his dictionary in 1828; it sold 2500 copies. In 1840, the second edition was published in two volumes.

In a general dictionary, each word may have multiple meanings. Some dictionaries include each separate meaning in the order of most common usage while others list definitions in historical order, with the oldest usage first.[20]

The simplest dictionary, a defining dictionary, provides a core glossary of the simplest meanings of the simplest concepts. From these, other concepts can be explained and defined, in particular for those who are first learning a language. In English, the commercial defining dictionaries typically include only one or two meanings of under 2000 words. With these, the rest of English, and even the 4000 most common English idioms and metaphors, can be defined.

Large 20th-century dictionaries such as the Oxford English Dictionary (OED) and Webster's Third are descriptive, and attempt to describe the actual use of words. Most dictionaries of English now apply the descriptive method to a word's definition, and then, outside of the definition itself, and information alerting readers to attitudes which may influence their choices on words often considered vulgar, offensive, erroneous, or easily confused.[23]Merriam-Webster is subtle, only adding italicized notations such as, sometimes offensive or stand (nonstandard). American Heritage goes further, discussing issues separately in numerous "usage notes." Encarta provides similar notes, but is more prescriptive, offering warnings and admonitions against the use of certain words considered by many to be offensive or illiterate, such as, "an offensive term for..." or "a taboo term meaning...".

Because of the widespread use of dictionaries in schools, and their acceptance by many as language authorities, their treatment of the language does affect usage to some degree, with even the most descriptive dictionaries providing conservative continuity. In the long run, however, the meanings of words in English are primarily determined by usage, and the language is being changed and created every day.[24] As Jorge Luis Borges says in the prologue to "El otro, el mismo": "It is often forgotten that (dictionaries) are artificial repositories, put together well after the languages they define. The roots of language are irrational and of a magical nature."

Sometimes the same dictionary can be descriptive in some domains and prescriptive in others. For example, according to Ghil'ad Zuckermann, the Oxford English-Hebrew Dictionary is "at war with itself": whereas its coverage (lexical items) and glosses (definitions) are descriptive and colloquial, its vocalization is prescriptive. This internal conflict results in absurd sentences such as hi taharóg otí kshetiré me asíti lamkhonít (she'll tear me apart when she sees what I've done to the car). Whereas hi taharóg otí, literally 'she will kill me', is colloquial, me (a variant of ma 'what') is archaic, resulting in a combination that is unutterable in real life.[25]

A historical dictionary is a specific kind of descriptive dictionary which describes the development of words and senses over time, usually using citations to original source material to support its conclusions.

In contrast to traditional dictionaries, which are designed to be used by human beings, dictionaries for natural language processing (NLP) are built to be used by computer programs. The final user is a human being but the direct user is a program. Such a dictionary does not need to be able to be printed on paper. The structure of the content is not linear, ordered entry by entry but has the form of a complex network (see Diathesis alternation). Because most of these dictionaries are used to control machine translations or cross-lingual information retrieval (CLIR) the content is usually multilingual and usually of huge size. In order to allow formalized exchange and merging of dictionaries, an ISO standard called Lexical Markup Framework (LMF) has been defined and used among the industrial and academic community.

In many languages, such as the English language, the pronunciation of some words is not apparent from their spelling. In these languages, dictionaries usually provide the pronunciation. For example, the definition for the word dictionary might be followed by the International Phonetic Alphabet spelling /ˈdɪkʃənɛri/. American English dictionaries often use their own pronunciation respelling systems with diacritics, for example dictionary is respelled as "dĭk′shə-nĕr′ē" in the American Heritage Dictionary.[26] The IPA is more commonly used within the British Commonwealth countries. Yet others use their own pronunciation respelling systems without diacritics: for example, dictionary may be respelled as DIK-shə-nerr-ee. Some online or electronic dictionaries provide audio recordings of words being spoken.

The age of the Internet brought online dictionaries to the desktop and, more recently, to the smart phone. David Skinner in 2013 noted that "Among the top ten lookups on Merriam-Webster Online at this moment are 'holistic, pragmatic, caveat, esoteric' and 'bourgeois.' Teaching users about words they don’t already know has been, historically, an aim of lexicography, and modern dictionaries do this well."[27]

There exist a number of websites which operate as online dictionaries, usually with a specialized focus. Some of them have exclusively user driven content, often consisting of neologisms. Some of the more notable examples include:

Guy Jean Forgue, "The Norm in American English," Revue Française d'Etudes Americaines, Nov 1983, Vol. 8 Issue 18, pp 451–461. An international appreciation of the importance of Webster's dictionaries in setting the norms of the English language.